The small basic C protein is a virulence factor that contributes to innate immunity evasion of Paramyxoviridae, but its mechanisms of action are incompletely understood. Focusing on the measles virus (MeV) C protein, we have shown that, first, it promotes accurate RNA synthesis by limiting the production of defective-interfering (DI) RNA. Second, we show here that C is recruited to sites of replication only when the large (L) subunit of the viral polymerase is present, suggesting the possibility of direct L-C protein interactions. We seek now to characterize the mechanisms of C protein action. The central hypothesis is that it is a co-factor that regulates RNA synthesis and enhances polymerase accuracy, thereby limiting the generation of DI RNA and the activation of innate immunity. The specific aims are, first, to assess how C interacts with L to promote polymerase accuracy. Second, to characterize how the C protein regulates viral genome replication and transcription. Third, to assess whether transcripts produced by copy-back DI RNAs are the main activator of the interferon response.